A load circuit for supplying electric power to loads such as a valve and a motor mounted on a vehicle includes a battery and semiconductor switches (MOSFET etc.) which are provided between the battery and the loads. The battery, the semiconductor switches and the loads are connected via conductors including the electric wires. Further, the load circuit is provided with a control circuit for turning on and off the semiconductor switches, whereby each of the semiconductor switches is turned on and off to thereby switch the operation of the load between a driving state and a stop state in accordance with a driving signal and a stop signal outputted from the control circuit.
Some of such the load circuits are each provided with a fuse for immediately interrupting the circuit to thereby protect a load, electric wires and a semiconductor switch etc. when an over-current flows through the load (see a patent document 1, for example).
FIG. 24 is an explanatory diagram schematically showing a load circuit of the related art. The power supply side terminal of each of loads 101 is connected to a battery VB via an ECU (an electronic control unit for an automobile) 102 and a junk box (J/B) 103.
The ECU 102 is provided with a plurality of semiconductor switches Tr1 such as MOSFETs etc. which are on-off controlled by a control IC 104. A fuse F1 is provided on the upstream side of each of the semiconductor switches Tr1, whereby electric wires W101 on the downstream side are protected by the fuses F1. In other words, electric wires each having a diameter (sectional area) capable of withstanding the interruption current of the fuse F1 are used as the electric wires W101 provided on the downstream side of the fuse F1. Further, the semiconductor switch Tr1 provided on the downstream side of the fuse F1 has the characteristics capable of withstanding the interruption current of the fuse F1.
Similarly, the J/B 103 is provided with fuses F2, whereby electric wires W102 on the downstream side are protected by the fuses F2.
In the case where valves are used as the loads 101, for example, the fuses F1, F2 degrade due to rush currents generated upon the turning-on of the valves and due to the repetition of the turning on/off operations of the valves. Thus, there may arise a case that the fuses F1, F2 are erroneously interrupted due to the aged degradation of the fuses F1, F2. In order to prevent the generation of such the trouble, the fuses are selected in view of margin with respect to a load current. That is, the fuses which interruption current is set to be slightly higher than usual are used. As a result, since it is necessary to use the electric wires and the semiconductor switches each adaptable to the characteristics of the fuse set in view of the margin, it becomes difficult to reduce the diameters of the electric wires and miniaturize the semiconductor switches used in the load circuit.